Field of the Invention
The invention relates in general to a capacitive sensor structure, a circuit structure with a capacitive sensor and a package structure of a capacitive sensor, and more particularly, to a capacitive sensor structure, a circuit structure with a capacitive sensor and a package structure of a capacitive sensor applied to fingerprint recognition.
Description of the Related Art
With the popularity of portable devices, e.g., smart phones carried by almost everyone, a substantial amount of important information is integrated and carried therein. Thus, access security of portable devices has become highly concerned by users. Biometrics technologies, which use biological characteristics to verify the real identity of a user, are technical means under popularization. Biometrics technologies include face recognition, iris recognition, vein recognition and fingerprint recognition. Among the above, fingerprint recognition is the most common and low-cost technology. As human fingerprints almost do not change after adulthood and are unique to individuals, the chances of completely identical fingerprints are almost next to zero. Thus, fingerprints are very suitable for verifying a real user identity.
A fingerprint sensor commonly refers to a sensing element capable of converting a fingerprint structure to an electronic signal. The resolution of a fingerprint generally used in the fingerprint recognition technology needs to be capable of distinguishing two adjacent ridges of a fingerprint. Two ridges are spaced by about 300 μm to 500 μm, and a height difference between a ridge and a valley is about 100 μm to 400 μm. To correctly perform the recognition, a fingerprint image captured by a fingerprint sensor usually requires an area of 1 cm2 and a resolution of 500 dpi.
Fingerprint sensors can be categorized into optical and capacitive types. Earlier fingerprint sensor technologies are mostly optical types, which collect digital grayscale images of ridges and valleys using capacitor coupling devices (CCD) or complementary metal oxide semiconductor (CMOS) image sensors. However, when fingerprint recognition is performed using optical fingerprint sensors, a higher recognition error may occur if a finger contains a slight scar or dust.
On the other hand, a capacitive fingerprint sensor is mainly formed by a sensor circuit and a controller circuit. The sensor circuit is a capacitor array formed by sensing electrodes. The controller circuit drives the sensor circuit and reads a capacitance change in the sensor circuit. The sensor circuit and the controller circuit of a conventional capacitive fingerprint sensor are both implemented by the same silicon chip, which is further provided with an insulation layer at a surface thereof. When a finger is placed on the capacitive fingerprint sensor, the distances from the ridges and valleys of the fingerprint to the sensing electrodes are different; hence, the ridges and valleys of the fingerprint generate different sensing capacitance values. As previously described, the fingerprint image captured by a fingerprint sensor usually requires an area of 1 cm2. That is to say, a silicon chip implementing a conventional capacitive fingerprint sensor also requires an area of 1 cm2. However, the area of a semiconductor chip and multiple mask high-end fabrication processes needed for completing the controller circuit contribute a major part of the cost of a circuit chip, meaning that conventional means for completing the silicon chip of a fingerprint sensor may result in an excessively large cost load. Therefore, there is a need for a capacitive fingerprint sensor applicable to a portable device or wearable device. It is a primary object of the present invention to solve the above issue.